1. Field of the Invention
The present invention relates to a hydrogen storage tank which is mainly used as a hydrogen fuel tank for an automobile, and in particular, to a hybrid type hydrogen storage tank where a cartridge is filled with a hydrogen occluding substance and contained within a tank.
2. Description of the Related Art
In recent years, together with global warming, carbon dioxide emitted from vehicles has become a problem, and efforts have been made to reduce the amount of carbon dioxide emissions. Fuel battery automobiles use power which is generated through an electrochemical reaction between hydrogen and oxygen as an energy source and does not emit carbon dioxide in the reaction process, and therefore, have been developed as automobiles for the next generation. Fuel battery automobiles are equipped with a hydrogen storage tank which works as a supply source of hydrogen, which is the fuel. It is important for the hydrogen storage tank to be safe, and it is required that the volume occupied by one hydrogen storage tank is as small as possible and the amount of hydrogen with which the hydrogen storage tank is filled is large. As the hydrogen storage tank which satisfies these requirements, a hydrogen storage tank in which a hydrogen occluding alloy is built in has been developed.
A hydrogen storage tank where a cylindrical container is filled with a hydrogen occluding alloy and a flange is developed as the opening of an end portion of the cylindrical container, for example, is disclosed (Patent Document 1).
In this case, the higher the pressure for filling the hydrogen gas is, the greater the amount of hydrogen that can be stored becomes, and airtightness involving the flange portion having a large diameter is not sufficient under high pressure, and therefore, there is a possibility of leakage.
In addition, a pressure container is disclosed where the liner is of a divided type, an opening of which the dimensions allow a unit for occluding hydrogen where a metal container is filled with a hydrogen occluding alloy to be inserted is provided, and measures have been taken in order to secure the sealing of the divided portion (see Patent Document 2). In this case, the smaller the diameter of the divided portion is, the smaller the force that is applied to the divided portion as a result of the pressure within the container is, and therefore, the easier it becomes to increase sealing, but the diameter of the divided portion cannot be made smaller than the outer diameter of the hydrogen occluding unit, and therefore, the possibility of leakage occurring due to high pressure at a certain level or higher, which exceeds the limit of the withstanding pressure of the sealing material or the like, becomes high.
Furthermore, more attention should be paid to the divided type liner because leakage may still occur in the sealed portion even if sealing can be secured in comparison with an integrally molded liner without sealing, and therefore, it is desirable to use an integrally molded liner, if possible.
In contrast, a hydrogen storage tank is disclosed where a manufacturing method for a gas storage tank, according to which drawing is carried out on the outer wall material (tank container) around the opening, is adopted after the outer wall material contains a filled portion which is filled with a hydrogen occluding alloy (heat exchanger), and thereby, the size of the opening of the outer wall material is large enough not to hinder the operation of the containment of the filled portion within the outer wall material, and the size of the opening of the outer wall material (opening for connection) is small enough to make it easy to secure airtightness of the tank so that the tank can bear the gas pressure from the inside when a gas is stored (see Patent Document 3).
According to this document, an integrally molded tank made of a metal is manufactured in accordance with the following method. That is to say, an aluminum alloy in columnar form which is to become a tank container (tank container before processing) is prepared, and a heat exchanger which is filled with a hydrogen occluding alloy is contained within this column. At this time, a support is provided between the aluminum alloy (tank container before processing) and the heat exchanger in such a manner that the weight of the heat exchanger is supported by the aluminum alloy (tank container before processing) via the support.
Then, drawing (drawing around the openings) is carried out at the two ends of the aluminum alloy (tank container before processing) so that the openings at the two ends are reduced in size so as to be converted to the openings for connection. As a result, a tank container having small openings for connection (openings) at both ends is formed.
After that, heat treatment (heating and cooling) is carried out on the tank container so that the fatigue strength of the aluminum alloy is increased. That is to say, the fatigue strength should be increased through heat treatment because the internal pressure increases and decreases when the filling of the hydrogen occluding alloy with hydrogen and the releasing of hydrogen are repeated, and therefore, the tank container expands and contracts, and thus, metal fatigue gradually increases. After the heat treatment, a reinforcing layer is formed around the outer periphery of the tank container, and thus, a hydrogen storage tank is completed.
It is disclosed that a hydrogen storage tank manufactured in the above described procedure has a light weight and a high resistance to fatigue, and in addition, allows hydrogen to be stored under a pressure of no lower than 35 MPa, and thus, an excellent hydrogen storage container can be formed.    [Patent Document 1] Japanese Unexamined Patent Publication 2000-55300    [Patent Document 2] Japanese Unexamined Patent Publication 2004-270861    [Patent Document 3] Japanese Unexamined Patent Publication 2004-286177